Battery cells are used in a wide variety of electronic and electrical devices. Very often, individual battery cells are connected electrically in series to form battery packs having higher total voltage and higher total energy storage capacity than individual battery cells. One consequence of connecting battery cells in series is that all of the battery cells will experience the same electrical current. Therefore, the number of ampere-hours added or subtracted from every battery cell's charge level will be the same. However, individual battery cells may have different total capacity (in ampere hours). This is particularly true as the battery pack ages, because over time individual battery cells degrade: battery cell resistances tend to increase and battery cell total capacities tend to decrease. If this process occurs at different rates in different battery cells in a battery pack, then at some point in time one or more battery cells may limit battery pack performance. Due to differing total capacities and state-of-charge (SOC) levels (due, in part, to different self-discharge rates), some battery cells will encounter a lower operational design limit before other battery cells when utilizing the battery pack to power a load circuit, requiring the battery pack to stop powering the load circuit even when there is energy remaining in some battery cells in the battery pack. Similarly, some battery cells will encounter an upper operational design limit before other battery cells when charging the battery pack, requiring the battery pack to stop charging before all battery cells are charged to a desired upper operational design limit. This unnecessarily limits the total energy that can be stored by the battery pack.
To minimize the impact of differing total capacities and self-discharge rates, battery cells in battery packs are “balanced” or “equalized.” This process attempts to make all battery cell voltages in a battery pack equal, either at some pre-specified operating point (e.g., when the battery pack is fully charged), or continuously. Commonly, charge is drained from individual battery cells having voltage that is higher than the voltages of other battery cells in the battery pack. This operation is called “bucking” the battery cell(s). Charge is added to individual battery cells having voltage that is lower than the voltages of other battery cells in the battery pack. This operation is called “boosting” the battery cell(s). Equalization circuits may be designed to operate in buck-only mode, or in boost-only mode, or both buck and boost mode. A fourth option is to move charge from one battery cell to another—a process called “shuffling”—but has the same effect as equalizing in both buck and boost mode, so will not be considered separately from that here.
For some applications, the battery pack is either fully or substantially recharged frequently enough that it is sufficient to equalize battery cells during only the charging process itself. This has the advantage that energy is not depleted by the equalization process when the battery pack is disconnected from the charger and unable to recoup that energy from the charger. Furthermore, heat generated from equalization does not need to be dissipated while the battery pack is powering a load, so that further energy does not need to be wasted in thermal-management activities (powering fans, etc.). However, even during charging, it is undesirable to needlessly dissipate energy by incorrectly equalizing battery cells.
The most common approach to equalization is to compare the battery cell terminal voltages. Battery cells having terminal voltage higher than the others may be bucked; battery cells having terminal voltage lower than the others may be boosted. However, due in part to the nonlinear nature of the dynamics of battery cells, voltage equalization is not the optimal approach. Some battery cells would be bucked or boosted at some point during the charging process that should not have been bucked or boosted, requiring that other battery cells in the battery pack later be bucked or boosted to compensate. Energy is wasted. Instead, it is possible to predict which battery cells will limit the battery pack performance and use that information to compute which battery cells need to be equalized during the battery charging process, regardless of present battery cell terminal voltage. Energy will not be needlessly wasted due to bucking or boosting the wrong battery cells.
Accordingly, there is a need for a method to efficiently determine in a predictive sense which battery cells require bucking or boosting while charging a battery pack in order to optimize an equalization metric. This equalization metric may seek to minimize energy that is wasted by equalization. Additionally, the equalization metric may seek to maximize battery cell lifetime.